This paper provides a county-level investigation of the root causes of gun violence in the U.S. To guide our empirical analysis, we develop a simple theoretical model which suggests that firearm-related offenses in a given county increase with the number of illegal guns and decrease with social capital and police intensity. Using detailed panel data from the Federal Bureau of Investigation for the period 1986-2014, we find empirical evidence for the causal effects of illegal guns, social capital, and police intensity consistent with our theoretical predictions. Based on our analysis, we derive a range of policy recommendations.